After Adjusting for Other Factors That Contribute to Wage Differences

After adjusting for other factors that contribute to wage differences in the United States:

A) all economic studies indicate that wages of union workers are higher than nonunion workers.
B) all economic studies indicate that wages of union workers are essentially equal to those of nonunion workers.
C) some economic studies indicate that wages of union workers are higher than nonunion workers while others suggest that these wages are essentially equal.
D) most economic studies indicate that wages of nonunion workers are higher than union workers.